The Oil & Gas upstream production industry drills wells of ever increasing depth and complexity to find and produce raw hydrocarbons. The industry routinely uses steel pipe (Oil Country Tubular Goods) to protect the borehole (casing) and to control the fluids produced therein (tubing). Casing and tubing are made and transported in relatively short lengths and installed in the borehole one length at a time.
One way to drill a borehole more efficiently is to conserve borehole diameter. The most straightforward way to achieve this is to minimize the diameter of the pipe connections. Two types of premium oilfield connections, namely integral flush joints and slim diameter high performance connections have been utilized for these purposes. The outer diameter of a flush joint connection is substantially the same as the outside diameter of the body of the pipe. In other words, the connection is contained within the wall thickness of the pipe body.
It would be desirable to provide slim diameter and flush-type connections, as well as other connections, with improved compression ratings. To better understand compressive strength in flush and slim-diameter connections, some terminology should be established. Threads include a raised portion, the ridge or tooth, that fits into the recessed thread groove. The threadform is defined by a root, crest, stab flank, and load flank, each of which is actually a helically extending surface. As exemplified by FIG. 1, a profile (i.e., 2-dimensions) of the threadform is defined by a cross-sectional plane extending radially outward from a central axis of the tubular member or thread and includes a repeating “sequence” of ridge segments 10a, 10b, 10c and groove segments 12a, 12b, 12c, each ridge segment defined by the stab flank 14, crest 16 and load flank 18, and each groove segment defined by the load flank 18, root 20 and stab flank 14. Each groove segment of the profile is formed by a respective axial segment of the helical groove of the three-dimensional thread and each ridge segment of the profile is formed by a respective axial segment of the helical ridge of the three-dimensional thread.
Any given threadform will have one and only one “pitch line,” a term that is well known and understood by those of ordinary skill in the art of thread design. For example, ASME B1.7-2006 (entitled “Screw Threads: Nomenclature, Definitions, and Letter Symbols”), a well-known industry standard adopted by the American Society of Mechanical Engineers, defines the “pitch line” as the generator of the “pitch cone” for a tapered threadform. “Pitch cone” is defined in the same standard as “an imaginary cone of such apex angle and location of its vertex and axis that its surface would pass through a taper thread in such a manner as to make the axially measured widths of the thread ridge and thread groove equal.” Thus, the “pitch line” is an imaginary line 22 on the threadform profile that intersects the stab flank and the load flank such that the axial width WR of the thread ridge equals the axial width WT of the thread groove.
As shown in FIG. 1, the pitch line has traditionally been understood to be a single straight line intersecting the load flanks and stab flanks at specified points. This is also reflected in the ASME standard, which explains that the pitch cone—the imaginary three-dimensional surface generated by rotating the pitch line about the central axis of the thread—“is located equidistant between the sharp major and minor cones of a given thread.” As shown in FIG. 1, the major cone bounds the crest of an external thread taper, while the minor cone bounds the root. Thus, the pitch line has historically been understood not only to be a single straight line but also to be a single straight line located at the particular location shown in FIG. 1 and reflected in FIG. 44 of the ASME standard.
The load flank and the stab flank are traditionally angled to create clearances between the tooth and groove so the two members that comprise the thread can fit together initially and be assembled without damage. The stab flank angle as and load flank angle UL are taken as positive as illustrated in FIG. 1. The included angle a1 is the algebraic sum of the two angles.
Square threads have substantially no flank angle and therefore are desirable because they provide good tension and compression load transfer. As described in U.S. Pat. No. 6,322,110, square or near square threads may include at least one relieved surface on the stab flanks that extends from the crest to some point on the stab flank surface; i.e., a surface with a larger stab flank angle to create additional clearance for the load flanks during make-up of the connection. The larger angles(s) alleviate some of the large thread flank clearance concerns. The clearance between the load flanks is “transferred” to the stab flanks as the connection ends come in contact and further torque is applied. Further make-up of the connection may allow the stab flanks to come back in contact, but typically only creating a helical point or line of contact or substantive contact that is only able to absorb so much stress upon final make-up.
As described in the preferred embodiment of U.S. Pat. No. 6,322,110, multiple angles (i.e., relieved surfaces) are used on the stab flank. In the “stabbed” position, i.e., as the male (or pin) of one connection is initially placed into the female (or box) of the mating connection, these surfaces enable the stab-flank of the pin thread to rest on the stab flank of the box thread while the load flanks have sufficient clearance to allow thread engagement as the pin is rotated to be “made-up,” i.e., rotated towards the final, fully engaged position of the connection. Furthermore, the relieved surface(s) cause the threads to engage such that the clearance between the load flanks is reduced during make-up because certain of the surfaces acted as a cam or inclined plane to reduce the clearance in certain parts of the thread. However, it is connection engagement (i.e., interaction between parts of the connection other than the threads (such as a metal seal)) that actually halts the forward progress of the threaded connection and will causes the contact within the threaded portion of the connection to shift from the stab flank to the load flank. This same movement shifts the existing clearance from the load flank to the stab flank. Make-up is achieved as the threads are driven together by applied torque which rotates the pin member, forcing the pin load-flank to move relative to the box load-flank. The shape of the stab flanks are such that as the threads reach final position, i.e., full make-up, the pin and box threads make two-dimensional point contact at the pitch-line.
In U.S. Pat. No. 6,322,110 the pitch line of the threadform is a straight line that produces a pitch cone when rotated about the center axis of the tubular member or thread. The pitch line is located equidistant between the root and crest along each of the stab flank and the load flank, which is standard for tubular connections. Controlled by tolerance limitations within the manufacturing process, the actual intersections of the pitch line on the stab flanks at full make-up may have a small clearance, surface contact, or a slight interference fit. However, the use of a threadform with a traditional straight pitch line is the only configuration shown in the '110 patent. This is shown by FIG. 5B of the '110 patent and the corresponding discussion at column 11, lines 41-56 (hereby incorporated by reference), which shows that pitch line 56 is a straight line, as generally understood in the art at the time. Such a configuration will limit the area of contact that can be created between the stab flanks of the pin and box members.